In the rubber products particularly requiring the strength such as tires for automobiles, conveyor belts, hoses and the like, a composite material formed by covering a metal reinforcing material such as a steel cord with a rubber composition is used for the purpose of reinforcing rubber to improve the strength and durability. In order that the rubber-metal composite material develops a high reinforcing effect to provide a reliability, a stable adhesiveness not depending on conditions of mixing, compounding, storage and so on is required between the rubber and the metal reinforcing material. In order to obtain such a composite material, there is widely used a so-called direct vulcanization adhesion, in which the metal reinforcing materials such as steel cords or the like plated with zinc, brass or the like are embedded in the rubber composition containing sulfur and adhered thereto at the same time as the vulcanization of rubber during the vulcanization by heating. Until now, there are made various investigations for improving the adhesiveness, particularly the humidity-aged adhesion between the rubber and the metal reinforcing material in the direct vulcanization adhesion.
For example, there is reported a rubber composition in which a resorcin or a resorcin-formaldehyde resin (hereinafter abbreviated as “RF resin”) obtained by the condensation of resorcin and formalin is compounded for improving the humidity-aged adhesion (see JP-A-2001-234140). The humidity-aged adhesion between the steel cord and the rubber is certainly improved by compounding the RF resin.
However, the resorcin or the RF resin is poor in the compatibility with rubber because the polarity is very high, and the precipitation of the resorcin or the RF resin or so-called blooming is caused in accordance with conditions of mixing, compounding, storage and so on, so that there is a fear of damaging an appearance of the rubber product. And also, when the rubber composition is stored over a long time of period ranging from the compounding to the vulcanization adhesion, a problem of deteriorating the adhesiveness is caused by blooming, so that it is necessary to rapidly conduct the vulcanization adhesion of the rubber composition containing the resorcin or the RF resin, which may detract the productivity of the rubber product.
Also, an adhesive material made from a mixed polyester having a resorcin skeleton with a weight average molecular weight of 3000-45000 is reported (see JP-A-7-118621). Although the mixed polyester having a high molecular weight is more compatible with the rubber as compared with the RF resin, the compatibility can not still be satisfied. Moreover, when the mixed polyester having a high molecular weight is compounded with rubber, the viscosity of the compounded rubber is increased to cause a problem of lowering the workability, and the humidity-aged adhesion is not sufficient.